Microfluidics generally refers to microfabricated devices, which are used for pumping, sampling, mixing, analyzing and dosing liquids. Prominent features thereof originate from the peculiar behavior that liquids exhibit at the micrometer length scale. Flow of liquids in microfluidics is typically laminar. Volumes well below one nanoliter can be reached by fabricating structures with lateral dimensions in the micrometer range. Reactions that are limited at large scales (by diffusion of reactants) can be accelerated. Finally, parallel streams of liquids can possibly be accurately and reproducibility controlled, allowing for chemical reactions and gradients to be made at liquid/liquid and liquid/solid interfaces. Microfluidics are accordingly used for various applications in life sciences. Microfluidic devices microfluidic are commonly called microfluidic chips.
For example, microfluidic-based bioassays require passing a liquid sample inside a microfluidic flow path. The flow conditions (volume passing and flow rate) are important as they impact the outcome of the assay. While several methods and devices for flowing liquids inside microfluidic flow paths have been developed, these methods either lack flexibility or operate with a limited type of samples and flow conditions.
Besides, the fabrication of microfluidic chips using semiconductor wafers such as Si wafers seems attractive: one may expect to benefit from a range of existing processes, as continuously developed in the past decades for integrated circuits, to obtain accurate microfluidic structures. However, contrary to what is done in semiconductor wafer processing, microfluidics generally have deep structures, i.e., around a few micrometer, up to 20 micrometers or even deeper. In many cases, 5 micrometers is already considered as a small depth in microfluidic applications because such a small depth can generate a large hydraulic resistance on a liquid and can block or become clogged with microbeads and particles, such a small depth can also be incompatible with samples containing cells. As a result, existing semiconductor wafer processes are challenged by, if not incompatible with the requirements needed for microfluidic chip fabrication both in terms of manufacturing processes and cost of fabrication.